Question

How to show that for a plane curve, \(\Large \kappa=\left |\frac{d\phi}{ds}\right|\) where \(\kappa\) is curvature, \(\phi\) is the angle between \(\mathbf{T}\) and \(\mathbf{i}\), and it is known that for a plane curve y=f(x),
\[\kappa=\frac{f''(x)}{\left(\sqrt{1+[f'(x)]^2}\right)^3}\]
??